1. Field of the Invention
The present invention generally relates to catalyst characterization and, more particularly, to determining the oxygen content of metal species in a heterogeneous catalyst.
2. Description of the Related Art
The performance of catalysts is highly dependent on their physical and chemical properties. However, it is often difficult to directly measure physical and chemical properties of supported metal species in catalysts, especially when the metal species are well dispersed. Catalyst developers therefore rely on sophisticated characterization techniques to determine the physical and chemical properties and performance characteristics of new catalyst designs.
One property that influences the performance of catalysts is the oxidation states of the catalyst metal species, as it is well understood that the catalytic activity of a metal is different from that of the same metal that is oxidized. Metal species in a heterogeneous catalyst can become oxidized during catalyst synthesis and/or processing. Various methods have been employed in the art to determine the oxygen content of metal species in a heterogeneous catalyst. The routine methods include X-ray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR).
XPS is also known as ESCA (Electron Spectroscopy for Chemical Analysis). It is a technique that measures the binding energies of the electrons in the atoms (a function of the type of atom and its environment) on a material surface. XPS spectra are obtained by irradiating a material with a beam of X-rays while simultaneously measuring the kinetic energy and the number of electrons that escape from the top 1 to 10 nm of the material being analyzed. Because the binding energies of the electrons in an atom depend on the chemical status (i.e., ionic or metallic), the binding energy spectra obtained from XPS can be used to derive the oxidation states of the targeted elements.
There are some limitations with XPS. One is the detection limit. It is difficult to obtain high quality XPS spectra in samples with low metal loadings, because the detection limit of XPS is typically >0.5 atomic percent (0.5 at %). For example, in a heterogeneous catalyst having palladium metal particles supported on alumina with a metal loading of 1 weight percent (1 wt %), the atomic loading of palladium is 0.2 at %, far below the detection limit of XPS. It would be very difficult to get high quality XPS spectra in such samples.
The XPS energy resolution is another limitation. In general, the energy resolution of an XPS instrument is ˜0.5 eV. However, the real XPS peak width depends on sample conditions. The peak width usually becomes broad for very small particles. The increased peak width is not desirable because it reduces the ability to distinguish the ionic state of targeted species, especially when the binding energy difference between ionic states is small. For example, the peak width in the case of small palladium particles can be as large as 2 eV, while the binding energy difference between the palladium metal (Pd0) and the Pd2+ ion is 1.2 eV. In this situation, it would be very difficult to identify the chemical state of palladium.
In TPR measurements, a certain amount of catalyst is loaded into a flow reactor. A gas mixture of hydrogen (or another reduction gas) in argon (or another inert gas) flows through the catalyst sample as the sample is heated from a low temperature to a higher temperature. The amount of reduction gas uptake during this process is measured and the amount of oxide is calculated based on the reduction stoichiometry. Various detectors can be used to measure the reduction gas uptake but thermal conductivity detector (TCD) is most often used.
Although TCDs are the most universal detectors available, it is well known that they are not very sensitive. A number of factors influence the detection limit of a TCD. The breadth of a given TCD peak is directly affected by the reduction kinetics. In general, for broad TPR peaks, the detection limit is ˜50 μmol of reduction gas. For sharp peaks, the detection limit can be as low as 10 μmol. As a way to increase the accuracy of data when using TCD in TPR, the amount of sample may need to be increased to boost the reduction gas consumption.